Figure 4. Offshore collection using boat and light to attract elvers.

Figure 4. Offshore collection using boat and light to attract elvers.

The elvers have to be trained to take formulated feeds. The common method is to lower baskets containing tubifex worms or minced meat of oyster and clam twice a day, in the early morning and in the late evening under a wooden enclosure lit by a 20-40 W lamp. Elvers do not feed at temperatures below 13°C, thus temperatures must be kept above 13°C. Feeding time is gradually shifted to daytime.

Initially, elvers may not eat. In the long run, however, they learn to eat and eventually, attain rapid growth. After 100-150 days, a size of 100-200 fingerlings/kg is reached.

Feeding Management

Eels are carnivorous fish and thus require high animal protein input. The more animal protein that can be provided, the greater the eels' weight will be. Most eel farmers feed their eels with formulated feeds that are chiefly made of fish meal with added carbohydrates, minerals, vitamins, and other ingredients. Due to the high cost of these feeds, most eel farms supplement these with raw fish such as mackerel, sardine, and anchovy. Trash fish or scraps that are minced using electric grinders are also used widely as feeds. Eels, however, convert formulated feeds more efficiently than they convert raw fish.

Feeding is done once a day, at about 7:00 a.m. during summer and about 10:00 a.m. in winter. The usual feeding rate is 5-15% of the total weight of the eel for minced trash fish and 1.0-3.5% for formulated feeds (11). Nevertheless, factors such as the previous day's feeding condition and water and air temperature as well as water quality are considered when determining the feeding rate. Ideally, feeds should be consumed within 20-30 min.

Precautions must be taken to insure that no unnecessary organic debris are allowed to fall into the ponds. Thus eel farmers do not just broadcast the feeds into the ponds. Instead, a special feeding platform, usually shaded, within the pond is designated for feeding, and the eels learn to swim toward the area each feeding time. Feeding areas are preferably located where the oxygen level is high as this encourages the eels to eat well. Formulated feed is lowered on a perforated tray suspended just under the surface and lifted after feeding is done (Fig. 5). Raw fish, dipped for a few minutes in boiling water to soften the skin, are threaded through the eyes and also lowered and raised once eels have eaten all the flesh. Thus the only organic matter that goes into the pond is that which the eels eat.

Pond and Water Management

Growout ponds are either rectangular or square. In Japan, adult eels are raised in several types of ponds that make use of efficient drainage with water temperatures of about 28°C. Until recently, these eel ponds were between 5,00020,000 m2 in area. However, the practice of intensive culture has reduced pond size to about 500-1,000 m2, and is expected to be further reduced in the future (10).

In Taiwan, two types of eel ponds are used, namely, the hard pond and the soft pond (Fig. 6). The hard pond has a concrete or red brick bottom and dikes and is small (1,0001,650 m2 and water depth of 0.8-1.2 m). The soft pond, on the other hand, has a mud bottom and concrete or brick dikes and is generally large (0.6-1.0 ha and water depth

Figure 5. Eels feeding on formulated feed paste from a perforated tray lowered into the pond. The tray is lifted out once the eels stop feeding to avoid scraps fouling the water.
Figure 6. Soft ponds.

of 2.0-3.0 m). Recently, some farmers have reverted back to using earthen ponds (Fig. 7).

Most eel farms have many ponds of different sizes because eels of different sizes must be sorted to maintain uniform growth. The eels that grow fast are separated from those that grow slow and are transferred to larger ponds.

Figure 7. Earthen ponds.

Pond water quality is also an important aspect of eel culture because it could affect the health and growth of the cultured eels. A well-maintained pond could spell the difference between healthy and fast-growing eels and poor quality ones.

Pond water color can also be used to gauge water quality. The most favorable color is green because it can show the presence of zooplanktons, phytoplanktons, ideal pH value, high dissolved oxygen concentration, and other factors affecting water quality.

Eel ponds require large amounts of fresh water and are thus generally located in areas with good water supply both quantitatively and qualitatively. Water is kept free from pollution and within pH values of 6.5-8.0. Temperature is kept above 13°C because eels stop eating below this temperature. Oxygen, which is considered the most critical water characteristic, must be maintained above 1 mL/L because eels cannot sustain life below this level.

To ensure and improve the water quality in eel ponds, various methods may be adopted. These would include allowing more water to flow into the pond to increase the rate of water exchange, installing aerators to increase oxygen supply, and liming to improve water quality. Bottom sediments are also removed after harvest.

Sleeping Sanctuary

Sleeping Sanctuary

Salvation For The Sleep Deprived The Ultimate Guide To Sleeping, Napping, Resting And  Restoring Your Energy. Of the many things that we do just instinctively and do not give much  of a thought to, sleep is probably the most prominent one. Most of us sleep only because we have to. We sleep because we cannot stay awake all 24 hours in the day.

Get My Free Ebook

Post a comment